To reduce the number of line pairs between a central office and individual subscriber phones, various kinds of pair-gain systems have been developed. Remote concentrators have been used which connect a number of subcriber phones to a smaller number of line pairs going to the central office. The shared line pairs are then expanded back to the full complement of subscriber line pairs at the central office. These concentrators may use some priority scheme for allocating the available channels between the remote station and the central office among the larger number of subscriber phones.
Because the standard central office No. 5 crossbar switching system already includes a concentrating portion, conventional remote switching systems which provide concentration and expansion are inherently redundant. Prior attempts have been made to take advantage of the concentrating aspect of the central office switching system by physically displacing the concentrating portion of the central office to a remote location. Such a system is described, for example, in U.S. Pat. No. 4,038,498. This patent shows an arrangement which takes advantage of the concentrating effect in the link frames of a standard Western Electric No. 5 crossbar switching system of the type described in U.S. Pat. No. 2,585,904. The input stage of the standard line link frame crossbar switch selectively connects a group of up to 50 subscriber lines connected to the vertical lines of the crossbar switch to any one of a group of 10 horizontal lines of the crossbar switch, thus providing an inherent concentrator function. The output stage of the same line link frame crossbar switch connects the 10 horizontal lines to 10 vertical lines which in turn connect to a corresponding number of junctors extending to the trunk line frames of the No. 5 crossbar switching system. U.S. Pat. No. 4,038,498 provides an arrangement for physically separating the input stage with its concentration from the subscriber lines to the horizontal lines of the crossbar switch, to a remote station.
However, the basic design of the No. 5 crossbar switch makes it relatively uneconomical to detach the line link frame or any part of it for relocation to a remote site. The presence of other control circuits in the line link frame, such as portions of the marker circuit, greatly increases the complexity in cost of any interface system which is required to physically relocate any part of the line link frame to a remote location.